The invention relates to a two-wire time-division multiplex method of full duplex transmission between a central or base station and a substation.
The two-wire time-division multiplex method of full duplex transmission of bursts between a base station and a substation is further disclosed in "Digitale Vermittlungstechnik" by H. Besier et al, published by R. Oldenbourg, Munich-Vienna, 1981, pages 180 to 182.
The time-division multiplex transmission is effected by forming time frames, while the structure of the time frames is influenced by the time-division multiplex method implemented for separating the direction. For a reduction of the required modulation rate the blocklength (number of bits) within a time-division data block can be chosen to be largest possible. Usually two PCM words are comprised in a single time frame. As this time frame likewise constitutes a time-division data burst, no frame synchronizing information is required. The directional separation is realized in that transmitting and receiving on both sides of the transmission path does not take place simultaneously but alternately.
From the central station (exchange) an information burst is transmitted periodically. This block, containing the information of a multiplex frame, is received by the substation (terminal station) after the signal propogation delay time. After a protection interval, whose duration is fixed or adapted to the delay time, the substation starts transmitting its information burst of the same duration. This block is received by the base station after the signal propogation delay time. Then a specific waiting time elapses, until the central station again starts transmitting its next information burst. Thus the fixed cycle time results from the sum of twice the duration of the information burst, twice the delay time, the protection interval and the waiting time, while in accordance with the structure of the multiplex frame the cycle time has a value of 125 .mu.s, corresponding to a frame repetition rate of 8 kHz.
On page 182 it is disclosed that the maximum permissible line length with a useful bit rate of 80 kbits/s and a transmission bit rate of 256 kbits/s is approximately 4 kilometers. For extending the range at least two bursts are designed to be combined in one multiplex frame and jointly transmitted, so as to save delay times which necessitate higher intermediate storage costs. In this case the cycle time is 250 .mu.s, corresponding to a frame repetition rate of 4 kHz.
In addition, from the German Patent DE-OS 31 45 126 a data transmission arrangement for full-duplex transmission is known, in which the transmission is effected in a bit-by-bit mode instead of in bursts. Also in bit-by-bit transmission the achievable range depends on the signal delay time on the line. In the two-wire time-division multiplex method known from the above Patent a transmission clock is generated in the central station and a substation clock synchronizable with the transmission clock is generated in the substation. In the first half of the clock period of the transmission clock a signal transmission takes place from the central station to the substation and in the second half a signal transmission takes place from the substation to the central station. The signals are sampled in the substation after a quarter of the clock period of the transmission clock, thus at the instant of the largest eye opening. Furthermore, the clock in the substation tracks the received data flow through a phase locked loop, resulting in a phase shift between transmission clock and substation clock, which corresponds to the signal delay time on the line. The sampling of the data in the central station takes place shortly before the end of the respective bit period of the transmission clock. The proper operation of this two-wire time-division multiplex method known from DE-OS 31 45 126 requires that the signal transmitted from the substation to the central station be received at the central station before the end of the relevant bit period.
The limit value for the achievable range is reached when the signal propogation delay time on the line amounts to a quarter of the bit period.